6. COSMOLOGICAL IMPLICATIONS OF SECULAR EVOLUTION

The qualitative arguments in Sections 2 -
4 and the star-formation measures
in Section 5 imply that secular evolution
increases bulge-to-total luminosity ratios B / T. How much
evolution along the Hubble sequence is plausible?

This question is too important to be postponed, but we warn readers that the
results of this section are very uncertain. To address the question, we
compare predicted B / T ratios with the distribution of
values observed by
Simien & de
Vaucouleurs (1986).
They decomposed the B-band surface brightness profiles of 98
galaxies to measure B / T
as a function of RC2 type. They added up all of the central light in
excess of the inward extrapolation of exponentials fitted to the outer
disks, so B / T measures the sum of bulge and pseudobulge
light. They found that B / T
is typically 2% in Sd galaxies, 9% in Sc galaxies, 16% in Sbc galaxies,
24% in Sb galaxies, and 41% in Sa galaxies. The scatter around these
values is large. In Sections 5.1 and
5.2, we estimate that
circumnuclear star-forming rings grow pseudobulges with masses of ~
109M. The
plausible range of these masses is also large, from ~ 107 to
1010M. The
total stellar masses of these galaxies are of order (1 to 5) ×
1010M.
Therefore secular evolution can reasonably have produced pseudobulges
with masses ranging from 0% to >10% of the total stellar masses of
the systems. This is comparable to the B / T value in Sc
galaxies, consistent with our conclusion that Scs contain pseudobulges.
Evolution of one Hubble stage - e.g., from Sd to Sc - is plausible at the
late end of the Hubble sequence. Evolution from Sc to Sbc is also
plausible.

However, it is less easy for secular processes to form the more massive
bulges of S0-Sb galaxies. The B / T ratio in these
galaxies is large, and the galaxies tend to be very massive. Total bulge
masses are at least 1010 - 1011M.
The evidence from stellar populations
(Section 8.1) is that the stars
in these bulges formed quickly and long ago. We conclude that the stars
in these bulges formed mostly during hierarchical clustering. That is,
S0-Sb galaxies mainly contain classical bulges. Secular processes can
contribute modestly to the growth of classical bulges, but evolution by
half of a Hubble stage is expected to be unusual. Based on present star
formation rates, Sab galaxies like NGC 4736 that have dominant
pseudobulges should be rare.

In fact, they are not
extremely rare, and even some S0s have pseudobulges. The above
estimates are lower limits for at least two reasons. First, disk
galaxies presumably contained more gas in the past. Second, some
secular processes, such as buckling instabilities, do not depend on
concurrent star formation. They elevate pre-existing disk stars into
the pseudobulge.

Finally, we make a preliminary comparison of the relative importance of
secular and merger-induced star formation in the present universe. As shown
above, it is plausible that most of the stars in Sc-Sm (pseudo)bulges
and a significant fraction of the stars in Sb-Sbc (pseudo)bulges formed
as a result of secular evolution. Given the relative numbers of early-
and late-type galaxies, classical bulges and pseudobulges are not very
different in number. However, the masses of (mostly) classical bulges
in early-type galaxies are at least one to two orders of magnitude
larger than the masses of pseudobulges in late-type spirals. Integrated
over the history of the universe, star formation caused by secular
processes has contributed at most a few percent of bulge stars. The
vast majority of bulge stars are believed to have formed in collapse
and merger events.

However, most of these events
occurred in the distant past. Observations show that merger rates
increase dramatically with increasing cosmic lookback time
(Patton et al. 2002,
Conselice et
al. 2003).
The fractional contribution of galaxy interactions and mergers to the
total present-day SFR in the universe has been estimated by many
workers, starting with
Larson & Tinsley
(1978).
Kennicutt et
al. (1987)
estimated that 6% ± 3% of current star formation is induced by
galaxy-galaxy interactions. This contains two, partly compensating
uncertainties. It underestimates dust-extincted star formation in
bulges, but it overestimates bulge star formation because it includes a
contribution from distant tidal interactions as well as mergers. How
does this value compare to the contribution from secular evolution? In
Section 5.3, we note that ~ 10%
of intermediate-type spirals contain circumnuclear disks or rings and
that their star formation accounts for 10-80% of the current SFR in
those galaxies. These same intermediate-type spirals dominate the
current total cosmic SFR
(Brinchmann et
al. 2003).
Combining these numbers suggests that a few percent of present-day star
formation is attributable to secular processes. Thus, galaxy mergers
and secular evolution produce comparable star formation in the present
universe. Both contributions are small compared with the dominant
source of star formation at z = 0, namely the quiescent star
formation in the disks of spiral and irregular galaxies. Nevertheless,
as argued in the Introduction, we live approximately at the epoch of
transition when secular processes are overtaking mergers as the primary
mechanism that forms stars in the central parts of galaxies. We
emphasize again that all the estimates in
Sections 6.1 and
6.2 are very uncertain.